In March 2015, an aircraft powered only by solar energy took off in an attempt to complete the first-ever solar powered round-the-world flight. The Solar Impulse 2 started its mammoth journey in Abu Dhabi, with the aim of flying over oceans and continents to get back to where it started. The whole flight is expected to take around 500 hours or just over 20 days, but this is being split into approximately ten separate flights spread over five months.

However, in July 2015, the aircraft was grounded in Hawaii, 1,955km into its epic 34,998km) journey, after it suffered battery problems caused by high temperatures in the tropics. Now, after some damage repairs, Solar Impulse 2 is ready to fly again, and will resume its world record attempt at 15:00 UTC on 21 April 2016.

Despite its unlimited energy supply, the aircraft’s ability to complete the mission without stopping is prevented by the pilot’s need for rest and a limited space for food supplies. Taking turns at the controls will be Solar Impulse initiator and chairman Bertrand Piccard and cofounder and CEO André Borschberg. Flying over the Pacific and Atlantic Oceans will require the pilots to stay in the air for up to five days and nights at a time, putting their endurance to the test.

The Solar Impulse 2 was designed and built with help from simulation software produced by Dassault Systèmes. The aircraft works by turning sunlight into electric energy using the vast number of solar cells stretched across its enormous 72-metre (236-foot) wingspan. Despite being wider than a Boeing 747 Jumbo Jet, the whole aircraft weighs only 2,300 kilograms (5,071 pounds) – less than a Land Rover Discovery jeep – helping it climb to a maximum altitude of 8,500 metres (27,890 feet). At night it will drop to 1,500 metres (4,920 feet) to conserve as much of the energy stored in the four batteries as possible. The large wingspan not only makes the aircraft very difficult to manoeuvre, but also means it is very sensitive to turbulence. If it tilts (banks) by more than 5 degrees it could go into a spin, but the pilot is notified by a vibration alert if the maximum bank angle is exceeded. To avoid turbulence and winds of more than seven knots (13 kilometres [eight miles] per hour), all takeoffs and landings will be scheduled at night.

Solar Impulse 2 Facts

The aircraft has maximum power of 70hp (52.2kW) and can reach speeds of up to 140km/h (87mph) at maximum altitude.

The entire motor system is 94 per cent efficient, setting a new record thanks to newly developed materials and technologies.

The main structure is made from carbon-fibre sheets that are three times lighter than paper.

Energy is stored in four lithium polymer batteries, which weigh 633kg (1,396lb) – over a quarter of the aircraft’s total weight

The batteries can store 260Wh/kg and can be fully charged in just 3-4 hours when the aircraft is grounded.

The four motors each generate 17.5hp (13kW) of power, rotating the 4m (13.1ft)-diameter propellers to create thrust.

Inside the cockpit

The 3.8-square-metre (40.9-square-foot) cockpit will be each pilot’s home for up to five days and nights at a time. It can store the 2.4 kilograms (5.3 pounds) of food, 2.5 litres (0.7 gallons) of water and one litre (0.3 gallons) of sports drinks they will need to consume each day, plus enough oxygen to survive in the unpressurised cockpit.

Temperatures will fluctuate between +40 and -40 degrees Celsius (-40 degrees Fahrenheit) while in the air, so the cockpit is insulated with isolation foam, and the pilot’s clothing contains intelligent nylon fibres to stabilise their body temperature. The multipurpose seat contains the toilet, parachute and life raft, plus it can lie flat to allow the pilot to stretch their legs.

A matchbox-sized electrocardiogram will monitor the pilot’s fatigue and vigilance and a tailor-made autopilot system will monitor the aeroplane. The pilot will also have a vibration device fitted into their sleeves to alert them to any problems or anomalies.

72-hour flight simulation

In preparation for the round-the- world mission, both pilots have completed 72-hour stints in a flight simulator, recreating the conditions of the Solar Impulse 2 cockpit. This enabled them to test and evaluate their nutrition plan, toilet facilities and exercise regime to prevent deep vein thrombosis (DVT). They could also try out their rest strategy, which involves using relaxation techniques for the shorter flights (24 to 36 hours) over land and taking 15 to 20-minute micro-naps for the longer stretches over oceans. Self-hypnosis and meditation techniques will also help them to maintain concentration and vigilance and help the pilots fall asleep and wake up faster.

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